Strong Water Absorption in the Dayside Emission Spectrum of the Planet HD 189733b

TL;DR

This study reports the detection of strong water vapor absorption in the dayside emission spectrum of exoplanet HD 189733b, challenging previous non-detections and suggesting complex atmospheric dynamics and heat distribution.

Contribution

It provides the first high-quality mid-infrared emission spectrum showing water absorption in HD 189733b, highlighting potential atmospheric variability and the need for refined models.

Findings

Detection of strong water absorption features in the emission spectrum.

Significant differences from previous observations suggest atmospheric variability.

Weak heat redistribution from dayside to nightside inferred from models.

Abstract

Recent observations of the extrasolar planet HD 189733b did not reveal the presence of water in the emission spectrum of the planet. Yet models of such 'Hot Jupiter' planets predict an abundance of atmospheric water vapour. Validating and constraining these models is crucial for understanding the physics and chemistry of planetary atmospheres in extreme environments. Indications of the presence of water in the atmosphere of HD 189733b have recently been found in transmission spectra, where the planet's atmosphere selectively absorbs the light of the parent star, and in broadband photometry. Here we report on the detection of strong water absorption in a high signal-to-noise, mid-infrared emission spectrum of the planet itself. We find both a strong downturn in the flux ratio below 10 microns and discrete spectral features that are characteristic of strong absorption by water vapour. The differences between these and previous observations are significant and admit the possibility that predicted planetary-scale dynamical weather structures might alter the emission spectrum over time. Models that match the observed spectrum and the broadband photometry suggest that heat distribution from the dayside to the night side is weak. Reconciling this with the high night side temperature will require a better understanding of atmospheric circulation or possible additional energy sources.